Low-calorie diet prevents the development of mammary tumors in C3H mice and reduces circulating prolactin level, murine mammary tumor virus expression, and proliferation of mammary alveolar cells.

The effect of carlorie intake on the development of spontaneous mammary tumors in virgin C3H mice was studied. Only about 10% of the mice fed a low-calorie diet [10 kcal/day (1 kcal = 4.184 kJ)] since weaning developed mammary tumors, compared to about 60% of those mice that were reared on high-calorie diets (16 kcal/day or lab chow ad lib). In order to understand the mechanism by which a low-calorie diet decreases the occurrence of mammary tumors in mice, we compared the sex cycle, the amounts of circulating thyroid-stimulating hormone (thyrotropin), growth hormone, and prolactin, the production of type A and B virus particles in the mammary glands, and the morphology of the mammary glands of mice fed low- and high-calorie diets. The amount of serum prolactin and the synthesis of type A and B particles in mammary tissues of mice fed a low-calorie diet was markedly decreased compared to those of age-matched mice fed high-calorie diets. In addition, in young mice fed a low-calorie diet, there were fewer mammary alveolar lesions than in mice fed a high-calorie diet, although the size of the lesions was similar. However, in older mice fed the high-calorie diet, the number and size of these lesions were greater than in the mice raised on the low-calorie diet. The other factors that we studied were not affected by calorie restriction. Our findings suggest that the reduction in serum prolactin level, mammary tumor virus production, and proliferation of mammary alveolar lesions associated with dietary calorie restriction is responsible for lowering the incidence of mammary tumors in mice.